It is an object of this invention to provide a composition and method for bonding polyethylene terephthalate, polycarbonate and other transparent thermoplastic sheets to form optically clear laminants with good bond strength. In particular, it is desired that the cosmetic and physical integrity of these bonded laminants remain intact over an exposure of temperatures ranging from -40.degree. C. to 70.degree. C. and relative humidity of up to 90% at 40.degree. C.
Unimpregnated industrial plastic laminants laminated in sheet form and having window-like appearance have typically been bonded with dry film adhesives. Dry film adhesives provide greater ease in laminant assembly by virtue of their room temperature non-tacky surface and are less prone to air entrapment than their quasi-liquid and liquid resin counterparts. Heat and pressure are required to allow proper wetting of the adhesive to each substrate and, as such, laminating temperature is usually the limiting variable in selecting a dry film adhesive for bonding plastic sheets.
Heat distortion temperatures of the thermoplastic laminants in question are relatively low. For example, polycarbonate film has a distortion temperature of 135.degree. at 66 psi laminating pressure. For platen lamination processing, the dry film adhesive of choice should, therefore, obtain its properties in a temperature range of 70.degree. C. to 125.degree. C. This range is substantially reduced for hot nip roll laminating since the nip roll in contact with the plastic sheet must be substantially higher than the activation temperature of the adhesive; e.g., a 70.degree. C. to 90.degree. C. range. The lower temperature (70.degree. C.) assumes that no cross-linking between the adhesive and thermoplastic sheet is obtainable at that temperature, and the resultant thermoplastic bond must remain intact at or below 70.degree. C. as originally stated.
In our early efforts to find a commercial dry film adhesive meeting the above requirements we discovered surprisingly few candidates. Acrylics, polyvinyl butrol and polyurethane thermoplastic films made up the short list. All other laminating films were either of the wrong chemistry for adhesion such as vinyls and olefins, for example, or required excessive laminating temperatures such as epoxies and phenolics, for example. Of the three, only the polyurethanes provided adequate laminant properties. The acrylic dry films formed laminants with poor adhesion when flexed and polyvinyl butrol showed poor water resistance.
It is known that polyurethane dry films have been used for the manufacture of polycarbonate/acrylic sheet laminated aircraft windshields. Those based on aromatic diisocyanates and polyester-polyols are best suited for maximum adhesion. The use of polyurethane dry films, however, did not produce adequate peel strengths for our laminating applications. In the design of the invention, the chemical functionality of phenoxy resins was very desirable. The combination of pendant hydroxies, phenyl groups and high molecular weight gives tenuous adhesion to a large variety of substrates which is of most importance to our development. Such substrates includes; polycarbonate, polyethylene terephthalate, ABS, epoxy printed circuit boards which include copper clad epoxy impregnated glass fabric board such as those in various grades classified by National Electrical Manufacturers Association (NEMA), e.g., G-10, G-11, FR-2, FR-3, FR-4 and FR-5, and the like. Phenoxies have several disadvantages, however. The primary disadvantage is that phenoxies are extremely brittle and as such, have poor flexibility at room temperature and are subject to cohesive failure when used alone as an adhesive. We found the peel strength of high molecular weight phenoxies to be on the order of 0.25 lb/inch, failing cohesively. A second disadvantage is the relatively high softening point of the high molecular weight phenoxies which is approximately 100.degree. C. Finally, for casting purposes, high molecular weight phenoxies require solvents that can adversely effect the properties of the desired substrates. For example, a 25% solids solution of Union Carbide PKHH dissolved in tetrahydrofuran or similar solvent is required to cast phenoxy at a solution viscosity of 400 cps. This solution dissolves polycarbonate substrates. Since the composition of phenoxy and polycarbonate resins are so similar they dissolve in the same solvents, e.g., tetrahydrofuran, mesityl oxide, diacetone alcohol and dioxane. The large amount of solvent, typically 75% by weight, required to dissolve the very high molecular weight phenoxy resin solid to a coating viscosity of approximately 400 cps, when in contact with the polycarbonate substrate also dissolves the polycarbonate. This dissolving occurs very rapidly, making direct coating of phenoxy on polycarbonate impractical.
The solution for these problems is a polyurethane-phenoxy resin blend. The combination of approximately 20% high molecular weight phenoxy and approximately 80% high crystallinity polyurethane show a synergistic result in adhesion, cohesion, application and temperature properties, and additionally allow the use of solvents which do not adversely affect polycarbonate resin substrates.